Behaviors and interactions during hydrothermal carbonization of protein, cellulose and lignin

Proteins participation will inevitably reconstruct the conversion route of lignocellulosic components and the formation of N-containing hydrochar during hydrothermal carbonization process. Herein, the behaviors and interaction mechanisms of protein, and typical biomass model chemicals (i.e. cellulose and lignin) were in-depth investigated. Results showed that the interaction of the binary mixture led to a reduction of hydrochar yield, but increased nitrogen concentration in hydrochar. Nitrogenous heterocyclic structure of hydrochar increased due to the interaction of protein with cellulose/lignin via forming Schiff bases and melanoids, wherein the content graphitic and pyridine nitrogen in protein-lignin hydrochar substantially increased from 5.4 % and 2.3 % to 9.7 % and 25.8 %, respectively. The increased ID/IG value from 0.81 to 0.91 in cellulose-protein hydrochar indicated the concentration of fused aromatic rings (≥6) based on Raman analysis. Carbon defects and disordered structure in hydrochar dramatically grew with the occurrence of the Maillard reaction and aldol-condensation of the intermediates. The nitrogenous substance in the aqueous phase increased by 30 %, resulted from protein decomposition accelerated by the hydrolysis of cellulose. This study provides a full understanding towards the transformation process of hydrochar from protein-containing biomass.